I like to think Vash runs on the colder side of things.
It's how he's survived out in the deserts for so long wearing a long coat and and all those layers. Sure, he still gets overheated, but not nearly as quickly as humans do. His Plant DNA keeps things regulated.
He'll use this to his full advantage when with you. Sniffling and shivering and begging, begging you to come cuddle by the fire. He's so cold mayfly. And when you give in to his whining, he'll wrap all four limbs around you and just burrow into your neck with his nose and hum like it's the best thing this side of the desert.
Does it matter that he's actually quite warm to the touch? Of course not. And anyways, weren't you getting cold all the way over there?? Of course you were, now shhhh.